There are three known methods for lubricating rolling element bearings for machines operating in an environment not permitting the use of any lubricating oil, or grease:
(1) A thin lubricating film of, for example, molybdenum disulfide (MoS.sub.2), tungsten disulfide (WS.sub.2), or silver (Ag) is formed by sputtering, or otherwise, on the sliding surfaces of all or a pert of the inner and outer races, the retainer and balls, as typically described in Japanese Patent Unexamined Publication No. 55-57717, 58-113629 or 61-55410;
(2) A retainer is made of a sintered alloy produced by adding a solid lubricant, such as MoS.sub.2 or WS.sub.2, to a metal or alloy, and the retainer material is transferred by the friction between the retainer and balls to the balls, and from the balls to the inner and outer races, to form a thin lubricating film on each of the balls, and inner and outer races, as typically described in Japanese Patent Unexamined Publication No. 62-151539 or 63-246507; and
(3) A retainer is formed from a plastic material or a composite high-molecular material prepared by adding a solid lubricant to the plastic material, and the retainer material is transferred by the friction between the retainer and balls to the balls, and from the balls to the inner and outer races, to form a thin lubricating film on each of the balls, and inner and outer races. Typical examples are retainers formed from fluororesins known as Rulon E (tradename of NTN Rulon) and Duroid (tradename of Rogers Corp., U.S.A.). These retainers are formed from a mixture of a polytetrafluoroethylene resin (hereinafter referred to simply as "PTFE"), glass fiber and MoS.sub.2.
The preliminary formation of thin lubricating films as described in (1) above, however, generally causes a bearing to have a short life, since the frictional wear of the films results in the loss of their lubricating effect and the exposure of the metal as a base material, causing its seizure.
The retainer made of a sintered metal as described in (2) above has the drawbacks of being difficult to machine, making significant friction noise, brittleness, cracking during use, and being heavily worn when exposed to air.
A bearing including a retainer formed from PTFE reinforced by glass fiber as described in (3) above has the drawbacks of quickly becoming worn, generating so much heat as to be likely to cause a variation of torque during use, since the glass fibers projecting from the surface of the retainer scrape off the lubricant transferred from the retainer and roughen the surfaces of the balls.
According to the solution as proposed, for example, in Japanese Patent Unexamined Publication No. 62-261718, hydrofluoric acid is used to melt glass fibers away from the machined surfaces of the retainer. Hydrofluoric acid is, however, toxic and its handling calls for special care. Moreover, glass fibers are exposed sooner or later again on the retainer surfaces, because of the wear of PTFE, and exert an adverse effect on lubrication similar to no such treatment being given at all.
There are also known retainers formed from composite materials containing high-molecular materials other than PTFE. A retainer formed form thermally stable polyimides, however, has the drawback of producing a large amount of gas in a vacuum. Japanese Patent Publication No. 63-1989 describes a retainer formed from a composition comprising a thermoplastic polyether aromatic ketone, a fluororesin and aromatic polyamide fibers. This retainer, however, has the drawback of being unevenly worn due to a greatly varying coefficient of friction, and thereby causing a variation of torque and axial deflection.
The known retainers formed form composite high-molecular, or sintered materials get worn more rapidly in their portions on which a load bears, and fail to supply the lubricant uniformly to the inner and outer races, and to the balls. The bearings including such retainers have, therefore, the drawbacks of being unstable in torque, producing a large amount of dust, and having a short life.
Under these circumstances, it is an object of this invention to provide a rolling element bearing which has an improved degree of lubrication owing to the steady and uniform transfer of a lubrication, and has a low level of torque and a long life, produces only a small amount of dust and gas, makes only a slight noise, and is easy and inexpensive to manufacture.